MATLAB timer object pitfalls and poor usage

Question

I created a timer that executes every 0.1 seconds. It calls a function that reads data and then updates the property of an object. When I start the timer, MATLAB displays the "Busy" signal at the bottom of the command window. MATLAB becomes unresponsive and I cannot halt the timer using the stop() function. My only recourse is to use Ctrl-C.

I determined the problem to be the processing time of the timer callback function was longer than the calling period and I presume no other MATLAB code could squeak into the stack/queue. This makes me somewhat worried about relying upon timers. My goal is to continuously make measurements from several devices, store them in an object, and need MATLAB to do other things in between these measurements. Also, I cannot afford to miss a measurement.

I am creating an app that responds to user input and provides the user with real time information, so I chose a fast period thinking it would produce a snapping UX. Since I am committed to using MATLAB I could not think of a better way of implementing this functionality than using a timer object. So the first question is, do timer objects seem like the right tool for the job I describe above?

Second, if I am to use timer objects would someone please share their experience about common mistakes or pitfalls using timers? Or has someone any advice on how to best implement timer objects? Is there a practical limit to the number of timer objects that can be used simultaneously? What is the best way to determine the optimal frequency of a timer object?

Thanks!

Answer 1

I would think that 0.1 seconds is pushing it for reliability, especially if you have multiple timers going at once, and especially if you want a user interface to be responsive at the same time as well.

MATLAB is basically single-threaded. There are exceptions, for example the lower level math routines call BLAS in a multithreaded way that speeds them up a lot. You can also write MEX code in C that is multithreaded, and call that from MATLAB. But there's basically one true thread that all your code runs on.

Timer objects are also an exception in a way. When you create a timer object, underneath that there's a java timer object, which is running on a separate java thread. But when any of its callbacks fire, it calls back into MATLAB to execute them, which happens on the one true thread.

If you cannot afford to miss a measurement, you'll need to set the BusyMode property of your timers to queue rather than the default drop, which will mean that if any of the callbacks take longer than 0.1 seconds to execute, they will back up - and you have to fit in any user interface actions as well.

In addition, MATLAB doesn't (and couldn't) make any real guarantees about the precision of how fast or regularly the timer callbacks will execute. If Windows suddenly decides to run a virus scan, or update itself etc, MATLAB will lose priority and that will mess things up. If you were firing timers every 10 seconds, or even every 1 second, it's in all likelihood going to be roughly accurate. But if you're firing them every millisecond, you can't expect it to be reliable - you'd need a proper real time environment for that. 0.1 seconds seems borderline to me, and I'd expect its reliability to depend on what exactly you were doing in those 0.1 seconds, and what else was going on as well (and what computer you're running, as well).

To answer your last questions (max number of timers, optimal timer frequency etc) - there's no general answer, just try it and work out what happens with a range of values in your particular situation.

If it turns out that it's not reliable enough, you could either try:

1. Doing the data acquisition in another faster way (e.g. in C), maybe in a separate process, then calling that from MATLAB via MEX, perhaps with some sort of buffering to smooth things out.
2. Turning to some the other MathWorks products (e.g. Simulink, Simulink Coder, some of the System Toolboxes) that are designed for developing properly real time systems.

Hope that helps!